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Anemia in adolescent schoolgirls in Western Kenya

Leenstra, T.

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Download date: 11 Oct 2020

CHAPTER 4

Permethrin-Treated Bed Nets in the Prevention of Malaria and Anemia in Adolescent Schoolgirls in Western Kenya

Tjalling Leenstra, M Penelope A. Phi l l ips-Howard, '•' Simon K. Kar iuk i , ' . '

William A. Hawley, W Jane Allaii, '-3 Daniel H. Rosen , 1 S Aggrey J. O loo , '

Bernard L. Nahlen, '>3 Piet A. Kager,2 Feiko O. te r Kuile 'A3

1 Kenya Medical Research Inst i tu te , Cent re for Vector Biology and Control

Research, Kisumu, Kenya.

2 Depa r tm e n t of Infectious Diseases, Tropical Medicine & AIDS, Academic

Medical Center, University of Amsterdam, the Nether lands .

3 Division of Parasitic Diseases, National Center for Infectious Diseases,

Centers for Disease Control and Prevent ion, CDC, Atlanta, GA, USA.

American Journal of Tropical Medicine

and Hygiene 2003; 68 (4): 86-93

Abstract

The impact of insecticide (permethrin)-treated bed nets (ITNs) on the health of adolescent

schoolgirls was investigated during a community based, randomized, controlled trial of ITNs

in western Kenya. Two school-based cross-sectional surveys were conducted to determine

the prevalence of malaria and anemia in 644 schoolgirls aged 12-18 years old in a rural area

with intense perennial malaria transmission. In 12- and 13-year-old schoolgirls, ITNs were

associated with a reduced prevalence of all cause anemia (hemoglobin level <12 g/dL, 16.9%

versus 31.4%, adjusted odds ratio [OR] = 0.38, 95% confidence interval [CI] = 0 .21 , 0.69)

and a 0.34 g /dL (95% CI = 0.02, 0.66) increase in mean hemoglobin concentrations.

No beneficial effect on all cause anemia (adjusted OR = 0.79, 95% CI = 0.43, 1.45) or hemo­

globin concentrations (difference in mean 0.14 g/dL, 95% CI = -0.24, 0.53) was evident in

older girls. In all age groups, no effect was found on malaria parasite prevalence or density,

clinical malaria, all-cause morbidity, standard measures of nutritional status and growth, or the

use of antimalarials and other medication. ITNs approximately halved the prevalence of mild

anemia in young, school-attending, non-pregnant, adolescent girls, but had no impact in older

girls or on other malaria-associated morbidity or nutritional status.

Introduction

Several large-scale trials have shown that insecticide (permethrin)-treated bed nets (ITNs)

reduce all-cause morbidity and mortality in children less than five years of age in areas of sub-

Saharan Africa with high or low, but seasonal malaria transmission.1'5 More recently, trials in

areas of intense perennial transmission have been completed in western Kenya and Tanzania,

and demonstrate similar beneficial impacts.6-7The 1TN trials have predominantly focused on

pre-school children and pregnant women, the two main risk groups for severe disease caused

by malaria in sub-Saharan Africa. However, the impact of ITNs on the healdi of adolescents,

particularly in areas of intense and perennial malaria transmission, remains unknown.

In holoendemic malarious areas, such as our study site in western Kenya, immunity against

clinical malaria is acquired during die first few years of life.8'9 School-age children are at a

markedly reduced risk of the adverse consequences of malaria because most children who sur­

vive tiieir pre-school years have acquired sufficient immunity to prevent severe disease associated

with Plasmodium falciparum infection. Earlier studies in this age group have indicated that,

on average, up to 60% of young adolescents may be parasitemic at any time.8 '1 0 A recent study

in adolescent males in western Kenya suggests that further development of anti-parasite immu­

nity against malaria occurs during puberty. 10This implies that apart from accumulated recognition

of parasite variants through childhood, host development during puberty, possibly mediated

by adrenal-hormones, is required to attain adult levels of immunity against malaria.10 Despite

incomplete development of the immune system, die majority oi infections in adolescents is

controlled and associated with low parasite densities only, does not result in fever, and is thus

likely to remain undetected and untreated.8 Such low-density infections, however, have been

associated with an increased risk of severe anemia in children under two years of age in western

Kenya.11 It is not known if chronic low-density infection among older children and adolescents

results in similar hematological consequences, or if malaria in this age group is truly mostly

asymptomatic. Potential hematological adverse effects are particularly relevant for the health

of adolescent girls, where pre-pregnancy hemoglobin levels may be a major determinant of

the risk of anemia related morbidity and mortality during possible later teenage pregnancy.12

Chronic anemia, especially when associated widi concomitant micro-nutrient deficiencies, may

also affect the adolescent's physical performance, growth, as well as school performance and

attendance, although there is little evidence that chronic malaria-induced anemia in the absence

of iron deficiency leads to decreased productivity and performance.15

We conducted a study to determine the prevalence of and risk factors for anemia, malaria,

and malnutrition in adolescent schoolgirls. Aldiough this study was not originally designed to

determine the impact of ITNs, it was conducted in the same geographical location as a concurrent

population-based study to determine the impact of ITNs on mortality and morbidity in children

less then five years of age.14 We took the opportunity to assess whether ITNs would have any

Bed nets in the prevention of malaria and anemia 73

significant effect on clinical parameters in adolescent schoolgirls, particularly anemia.

Evaluation of the magnitude and risk factors associated with anemia in this age group will be

presented elsewhere.

Materials and Methods

Study site and population

This study was conducted at the ITN trial site in Rarieda Division (Asembo) in Bondo District,

located on the shores of Lake Victoria in Nyanza Province in western Kenya. The study site and

the resident population have been described in detail elsewhere.14 '15 Briefly, approximately

55,000 people live in Asembo in an area covering 200 km2. The population is widely dispersed.

They are culturally homogeneous, predominantly Luo subsistence farmers who practice some

animal husbandry. Generally, the rainfall pattern is bimodal, with a long rainy season between

March and May, and a short rainy season from October to December.14 Malaria is holoendemic

and since some rain falls in each month, transmission occurs throughout the year. More than

90% of malarial infections are due to P. falciparum, infection with P. malariae making up most

of the balance, along with an occasional P. ovale infection. The number of infective bites per

person varies substantially at household level, but is calculated as a crude yearly average to

range between 60 and 300 per year.16 Anopheles gambiae and An. funestus are responsible for

more than 90% of the transmission, with the remainder transmitted by An. arabiensis. The ITNs

have been shown to reduce transmission by up to 90%.1 7 High-grade chloroquine resistance is

widespread in the area.18

There are 58 primary schools in Asembo. Primary school starts at the age of five years and

teaches children for a total of eight years (standard 1 to standard 8). Tuition is charged each

trimester and parents decide when their child will start school. Until recently, some parents

have waited for dieir child to reach the age of seven before sending them to school. If children

fail exams or miss schooling, for example when unable to pay tuition, they may repeat years,

with the result that some children may remain at primary school right up to the age of 18.

Girls may drop out of school because of pregnancy or if they become orphaned. A concurrent

prospective school-based study of younger children from the same area saw a larger number of

girls than boys lost to follow-up in the course of two years, suggesting that girls are more likelv

to drop out of school.19

Bed net trial

Details of the randomized controlled trial of ITNs are presented elsewhere.14 '20 bed net (Siam-

dutch Mosquito Netting Co., Bangkok,Thailand) prc-impregnated widr the target dose of 0.5g

permethrin/m2 of netting were randomly distributed to half the villages in Asembo during the

fourth quarter of 1996. ITNs were re-treated twice annually. The trial was implemented from

January 1997 to March 1999. An ITN coverage rate of 1.46 persons per ITN was achieved, with

an overall compliance (persons observed to be sleeping under ITNs) rate of 72%.2 I Residents in

the control villages received ITNs in April 1999.

Study design

The school-based study used a multi-stage random sample design, with primary schools as the

first stage unit and schoolgirls as the second stage unit.22 Prior to randomization, information

on the number and size of schools in the study area was obtained from the district education

authority and entered into a computerized database. Information on the longitude and latitude

of each school was added to the database using mapping data obtained using global positioning

system (GPS) hardware and a geographic information system (ATLAS-GIS).25 Schools were

selected by random sampling proportional to size ranked by geographical location to allow for

equal distribution of the schools over the study area.24 Schools with less than 30 girls in the

relevant age category were joined with the closest neighboring school to form one school unit.

In each school unit, 30 girls aged 12—18 years of age were then randomly selected using the

computerized list. A total of 840 girls in 28 schools were selected.

Data collection

Two cross-sectional surveys were performed at 28 selected schools at the end of the two-year

ITN trial; 14 schools in October—November 1998 and 14 schools in February—March 1999.

At each survey, participants were interviewed by a female study nurse to document age, date

of birth, school standard, village of residence, and reported ITN use. Participants were asked

about their recent medical history, including febrile and non-febrile illnesses in the previous

month, menstrual history, and any recent medication. The study nurse performed a clinical

examination on all participants to measure height, weight, and axillary temperature. Sexual

development was also assessed, using a modified Tanner score based on breast development

onlv.25 Anthropometric measurements were performed according to standard procedures of

the World Health Organization.26 Weight was measured to the nearest 0.1 kg on a battery-

powered digital scale (Seca, Inc., Columbia, MD). Heights were measured to the nearest

0.1 cm using a wooden length-measuring board with sliding head.27 All measurements were

taken in duplicate and the mean computed. Z-scores for height-for-age were calculated using

Epilnfo2000 (Centers for Disease Control and Prevention, Atlanta, GA).The 1977/1985

U.S.-based National Centers for Health Statistics/World Health Organization reference data

were used.

A finger-prick blood sample was drawn into heparinized capillary tubes (250—500 (XL) for

hematology and parasitology. Hemoglobin concentrations were measured in the field using

a portable battery-powered photometer (HemoCue, AB, Angelholm, Sweden). A full blood

Bed nets in the prevention of malaria and anemia 75

count, including repeat hemoglobin, was determined the same afternoon using a Coulter

Counter (Coulter, Hialeh, FL).The hemoglobin concentrations assessed by Coulter Counter

were used in this analysis, but if a hemoglobin result was missing (n = 7 of 650), it was replaced

with the HemoCue reading. Blood slides were stained with Giemsa and examined for die pres­

ence of malaria parasites at a magnification of xl ,000. Parasites and leukocytes were counted in

the same fields until 300 leukocytes were counted. Parasite densities were estimated using an

assumed leukocyte count of 8 ,000/mm' of blood. Slides were considered negative if no asexual

parasites were found in 200 high-power ocular fields of the thick blood smear.

Study participants were asked to bring a fresh (<24 hours) stool and urine sample to be

examined for the presence of geohelmindis (hookworm, Ascaris lumbricoides,Trichuris trichiura

and Strongvloides stercoralis) and Schistosoma mansoni. Samples were stored at 4°C and

processed within 24 hours after collection. Stool was microscopically examined by concentration,

using a modification of the formol-ether and ethyl acetate techniques, and by Kato-Katz

methods.2 8 5 0 Urine was examined using a filtration-based concentration method."

Participants found to be anemic (hemoglobin level <1 2 g/dL) during survey at their school

were given iron supplementation.Those having a documented fever (axillary temperature

a 37.5°C) were given presumptive treatment with sulfadoxine-pyrimethamine (SP), antibiotics,

or both, as indicated. Intestinal helminth infections and schistosomiasis were treated widi

albendazol and praziquantel, respectively.

Definitions

Adolescence was defined as an age of 12—18 years. Girls with a hemoglobin less than 12 g/dL

and 7 g/dL were considered to have anemia and severe anemia, respectively.32 Malaria was

defined as the presence of asexual blood stage parasites in the blood smear (any species).

Clinical malaria was defined as a positive malaria smear with a concurrent axillary temperature

a 37.5°C. Body Mass Index (BMI)-for-age (measure of thinness) and height-for-age (measure

of stunting) were used to measure nutritional status. ! ï The BMI was calculated as weight

(kilograms) divided by height (meters) squared, and thinness was defined as a BMI below the

fifth percentile for age. ï 3 Stunting was defined as height-for-age Z-score less than —2 standard

deviations from the mean of a reference population." Age was calculated from date of birth

as reported, and when possible, date of birth was checked with school records. If the exact

day of birth was unknown (in 4 1 % of the participants), the 15th day of the month was used.

If month of birth was unknown (in 28%), the midpoint of the year of birth was used .

Statistical analysis

Analysis was done using the SAS system for Windows, version 8.01 (SAS, Inc., Cary, NC) and

Epi-Info2000 (Centers for Disease Control and Prevention, Atlanta, GA). SUDAAN software

(SAS callable version; Research Triangle Institute, Research Triangle Park, NC) was used to

allow for correlation among observations taken from the same village (cluster-unit). Use of

village as the cluster unit, as opposed to school, was based on the assumption that participants

coming from one village were more alike concerning risk factors of the main end points (malaria,

anemia, and malnutrition) than those attending the same school since malaria is more likely to

be acquired at the villages after sunset than during school hours. In addition, randomization of

the exposure variable of interest (ITNs) was based on village cluster. To maintain the assumption

of an equal probability sample, weighting was used to adjust for unequal cluster size resulting

from variation in the number of absentees or refusals between clusters.24

Differences in proportions between ITN and control groups were compared with the

Cochran-Mantel-Haenszel chi-square test (Table 1). Multivariate logistic and linear regression

were used to estimate the effect of ITN use on various laboratory and clinical endpoints using

the backward elimination approach to assess interaction and confounding.'4The presented

means for hemoglobin concentrations, parasite density and hcight-for-age Z-score are adjusted

for the co-variates using linear regression (Table 2).

Analysis suggested potential effect modification by age of the impact of ITNs on anemia

(P value of interaction term = 0.07). Results of the effect of ITNs are given stratified by age

(age x ITN interaction term in the model with two age categories: 12 and 1 3 year olds and

a 14 years old), as well as pooled for all study participants (main effect model without the

interaction term).

The analysis of ITN efficacy was based on the intent-to-treat principle; groups are compared

based on randomization status (reported village of residence) rather than reported bed net use.

Two-sided P values <0.05 were considered statistically significant.

Ethical clearance and informed consent

The ITN project was reviewed and approved by the institutional review boards of the Kenya

Medical Research Institute (Nairobi, Kenya) and the Centers for Disease Control and Prevention

(Atlanta, GA).The study of the prevalence and risk factors of anemia in adolescent schoolgirls

was approved bv the institutional review boards of the Kenya Medical Research Institute and

the Academic Medical Center, University of Amsterdam (Amsterdam, the Netherlands).

Written consent was obtained from the individual student and her parents.

Results

Of 840 schoolgirls randomized, 669 (79.6%) were enrolled (321 in survey 1 and 348 in survey 2).

The remainder had either moved out of the study area or was not present at survey (n = 55)

or consent was not obtained (n = 116). Of the 669 girls present, 25 were excluded from the

analvsis: 21 because further analvsis using date of birth revealed they were younger than 12

Bed nets in the prevention of malaria and anemia 77

years old (despite their reported age), three because they had a history of recent blood transfu­

sion (two from ITN and one from a control village), and one because the village of residence

(randomization status) was unknown. Age and sexual development of the 644 participants were

comparable by randomization status, except that more girls from control villages had started

menstruating (Table 1). There was no difference in the number of girls reporting heavv men­

struation (P = 0.771). Of 339 participants from ITN villages, 49 (14.6%) reported not using an

ITN, while 31 (9.7%) of 305 participants from control villages reported sleeping in a house

having an ITN. All of the latter ITNs were reported to be study bed nets (identifiable by die green

color) supplied by die ITN project to intervention villages. Unlike die study nets in intervention

villages, bed nets in control villages were not retreated routinelv with insecticide bv die study staff.

Table 1

Characterist ics o f t h e study popu la t ion overa l l a n d s t ra t i f i ed for age and randomiza t ion g r o u p

:.;-_ 12-13

I T U '

years

Cont ro l

N u m b e r o f Study Par t ic ipants 163 131

M a t u r i t y leve l ; no . (%) M o d i f i e d t a n n e r Breast s tage

Bl

B2

B3

B4

B5

50 (30.3)

50(32.1)

51 (30.8)

7 (4.0)

5 (2.8)

44 (34.6)

40 (30.8)

34 (25.4)

9(7.0)

3 (2.2)

14-18

Bed net rando

ITN

176

8 (4.9)

18(10.0)

53 (29.8)

47(26.1)

50(29.1)

j years

mizat ion grou

Control

174

8 (4.2)

19(11.0)

60 (33.0)

40 (23.7)

47 (28.0)

Ov«

P

ITN

339

58(17.1)

68 (20.6)

104(30.3)

54(15.5)

55(16.5)

•rai l

Cont ro l

Ï 0 5

52(17.2)

59 (19.5)

94 (29.8)

49(16.6)

50(17.0)

M e n s t r u a t i o n ; no. (%)

normal

heavy

Any H e l m i n t h in s too l ; no. {%}

Hookworm; no. (%)

Schistosoma mansoni; no. {%)

Trichuris trichiura; no. (%)

Ascaris lumbricoides; no. (%)

Strongyloides stercoralis; no. {%)

Schistosoma haematobium; no. (%)

9 (55 )

0

72(51.0)

19(13.2)

25 (18.3)

30(21.5)

32 (22.1)

1 (0.7)

1 (0.7)

11 (8.4)

1 10.8)

55(51 .2)

16(15.0)

11 (10.3)

22(21.0)

31 (27.6)

0

0

67 (36.7)

22(11.8)

64(42 .2)

18(11.8)

12(7.6)

23(15.5)

32(21.1)

0

1 (0.8)

78 (45.4)

21 (12.5)

60 (43.5)

21 (15.0)

13(10.6)

14(10.6)

32(21.4)

0

1 (0.7)

76(21.7)

22(6.1)

136(46.4)

37(12.5)

37(12.8)

53(18.4)

64(21.6)

1 (0.4)

2 (0.7)

89 (29.5)'

22 (7.5)

115(46.9)

37(15.0)

24(10.5)

36(15.1)

I - >

0

1 (0.4)

1 Significantly different (P <0.05).

2 ITN = insecticide-treated bed net group.

Parasitemia

SU

45

40 -

35 -

5 £ 30

Pre

vale

nce

O

UI

15 -

10

5 -

n

Odds ratio (95%) 0.60(0.31-1.15)

9 •

Odds ratio (95%) 1.09(0.62-1.92)

12&13 14-18

Age (years)

Anemia (Hb < 12 g/dL)

40 Odds ratio (95%)

0.38(0.21-0.69)

12&13

Odds ratio (95%)

0.79(0.43-1.45)

14-18

Age (years)

Figure 1. Effect of insecticide-treated bed net (ITNs) on the prevalence of malaria parasitemia (upper graph) and anemia (lower graph) in

adolescent schoolgirls in Kenya. Dark bars represent control villages and light bars represent ITN villages. Text boxes contain the odds ratios

195% confidence intervals [CIs]) adjusted for age, cross-sectional survey and use of antimalarials (upper graph), or for age, cross-sectional

survey, menstruation, helminth infections and use of antimalarials (lower graph). Hb = hemoglobin.

Bed nets in the prevention of malaria and anemia 79

Positive blood smears for malaria were detected in 27 .8% of all girls. The prevalence was

lower in 12—1 3-vear-old girls from ITN villages than in girls from control villages, but this dif­

ference was not statisticallv significant.There was no difference in older girls (Figure l) .The geo­

metric mean (95% confidence interval [CI]) parasite density was 209/mm J (1 50-290 /mm' ) ,

with no difference between ITN and control groups overall or between the two age strata (Table 2).

The prevalence of high-density parasitemia and clinical malaria were also similar (Table 3).

Table 2

1 Mean difference adjusted for: age, cross-sectiona! survey, menstruation, helminth infections and malaria medication use

2 Parasitemic cases included only

Overall, 2 1 . 1 % of the girls were anemic (hemoblobin level <12 g /dL) . Only one girl

(from an ITN village) had severe anemia (hemoglobin level <7 g/dL) . Girls in ITN villages

were less likely to be anemic than girls in control villages; adjusted odds ratio (95% CI) for all

age groups = 0.55 (0.34 0.89) (P = 0.016). Interaction term models suggested that this

beneficial effect of ITNs on anemia was dependent on the age of the girl (P value age x ITN

interaction term = 0.07). A significant effect of ITNs was apparent only in 12-1 3-year-old girls,

while no difference was observed in older girls (Figure 1). Similarly, the mean hemoglobin level

was significantly higher in 12—13-year-old girls from ITN villages compared with control vil­

lages (P = 0.04), but mere was no ITN-associated difference in older girls (P = 0.46) (Table 2).

The magnitude of the effect of ITNs on anemia with concurrent malaria parasitemia was greater

than on all-cause anemia (Table 3).

Table 3

Ef fec t o f i n s e c t i c i d e - t r e a t e d b e d ne t s o n s e c o n d a r y e n d p o i n t s u s i n g l o g i s t i c r e g r e s s i o n j

I l lness En t h e p r e v i o u s

All ages

12-13 years

| 14-18years

Fever p r e v i o u s m o n t h ,

All ages

12~13years

14-18 years

.:.

m o n t h ; n o

n o . (%)

Used a n y m e d i c a t i o n l a s t m o n t h ;

I All ages

12-13 years

14-18 years

C r u d e p r e v a l e n c e

Bed n e t r a n d o m i z a t i o n g r o u p '

ITN

H i s t o r y o

t%> j a H 223 (65,0)

107(64.8)

116(65.3)

173(50.3)

86(52.0)

87 (48.8)

n o . (%)

133(38.9)

71 (40.4)

62 (37.3)

Used a n t i m a l a r i a l s fas t m o n t h ; n o . (%)

| All ages

12-13 years

14-18 years

109(32.5)

53 (32.8)

56 (32.2)

C o n t r o l

i l l ness a n d m e d i c a t i o n u s e

197 (63 J )

81 (61.9)

116(65.1)

153(50.3)

63 (48.7)

90(51.6)

137(45.4)

78(45.1)

59(45.9)

88 (27.9)

34U4J2)

54 (30.8)

A d j u s t e d P r e v a l e n c e

O d d s R a t i o

1.07(0.76,1.50)

1.13(0.71,1.80)

1.01 (0.61, 1.67)

1.03(0.72, 1.46)

1.17(0.69,1.99)

0.89(0.56,1.41)

0.77(0.52,1.13)

0.71 (0.42, 1.19)

0.82(0.52,1.29) : :v • .• . . . :::: . . . . : : •

Vvy'iBirMi- TirrrY^ 1.23(0.81, 1.88)

1.51(0.83,2.75)

1.05(0.63,1.76)

Used t r a d i t i o n a l m e d i c i n e l a s t m o n t h ; n o . {%)

All ages

12-13 years

! 14-18 years

HI a t p r e s e n t a t i o n ; n o

All ages

12-13years

14-18 years

(%)

C l i n i c a l m a l a r i a ; no» ( % } J

; All ages

12-13 years

14-18 years

115(33.9)

62 (37.8)

53 (30.3)

C l i n i ca

3(1.0)

1 (0.7)

2(1.3)

2 (0.6)

1 (0.6)

1 (0.6)

132(43.4)

53 (39.3)

79(46.6)

a n d l a b o r a t o r y m e a s u r e s

13(4.6)

6(5.9)

j 7 (3.7)

;;• • • •'•:• • • : . • • ••"-. •:•.' .. . • . - . • " ; ' : ^ ' ; ; V , • ' • • ••m^.-\ - ••:.-.

6(2.3)

4 (3.6)

2(1.4)

0.68(0.43,1.09)

0.98(0.58,1.66)

0.50(0.29,0.85)

""mms-0.20 (0.05, 0.77)

0.11 (0.02,0.77)

0.32(0.05,2.14)

031 (0.06, 1.68)

0.21 (0.02, 2.03)

0.53 (0.06,4.79)

Bed nets in the prevention of malaria and anemia 81

Table 3 (continued)

Effect of insecticide-treated bed nets on secondary end points using logistic regression

Crude prevalence

Bed net randomization gro

ITN

Anemia and concurrent parasitemia; no. (%)

All ages 12(3.7)

14-18 years 7(4.2)

High density parasitemia ï 500/mm'; no. (%)

All ages 19(5.9)

12-13 years 10(6.5)

14-18 years 9(5.4)

Anthropometrics BMI < 5th percentile; no. (%)

All ages

12-13 years

14-18 years

Helght-for-Age <

All ages

12-13 years

14-18 years

-2SD; no. (%)

63 (18.5)

38 (23.1)

25 (14.3)

HH

39(11.7)

31 (18.8)

8(5.1)

Control

54(18.4)

32 (24.2)

22(13.3)

30(10.1)

24(18.6)

6 (3.6)

Adjusted Prevalence

Odds Ratio

(95% CI)2

' 22(13.3) J

30(10.1)

0.42(0.17,1.02)

0.21 (0.06,0.72)

0.83 (0.27, 2.56)

0.77 (0.33, 1.84)

0.76 (0.29, 2.00)

0.77 (0.25,2.33)

0.97(0.60,1.58)

0.89(0.50,1.59)

1.09(0.52,2.29)

1.11 (0.58,2.13)

0.96(0.50,1.84)

1.53(0.42,5.42)

H i

1 Crude prevalence.

2 Adjusted for co-variates: age, cross-sectional survey, menstruation, BMI < 5th percentile, and helminth infections

3 Positive malaria smear plus axillary temperature > 37.5°C

4 Compared to no parasitemia and parasitemia < 500/mm'

ITN = insecticide-treated bed net.

Few girls (2.7%) were considered ill by the study nurse at the time of the physical exami­

nation, although this proportion was significantly higher in the control group (P = 0.038)

(Table 3). The BMI was below the fifth percentile for age in 18.5% of participants, and 10.9%

had a height below - 2 SD for their age. ITNs had no effect on nutritional status (Tables 2 and 3).

A high proportion (64.4%) of the study participants reported illness in the month prior to

survey (Table 3). Among them, headache was the most frequently mentioned symptom (84%),

followed by abdominal pain (31%), fever/chills (1 3%), and cough (12%). Medication was

reportedly used by 71.4% of participants in the preceding month. Antimalarials were used bv

almost one-third of the girls (30.3%), of whom 186 (94.8%) reported taking chloroquine,

five (2.5%) used SP alone, four (1.9%) a combination of chloroquine with SP, one (0.4%) used

amodiaquine and one (0.4%) used quinine. Use of iron and folic acid supplementation was rare,

with just 5. 3% and 1.1% of participants reporting their use, respectively. Reported recent

illness, fever, use of traditional or conventional medication, and use of antimalarials in the pre­

vious month were equally likely in control and intervention villages (Table 3).

Analysis taking school clustering into account rather than village clustering, or analysis based

on reported ITN use rather dian intention to treat, did not alter the conclusions.

Discussion

In this study we explored die effect of ITNs on malaria-associated outcomes in adolescent school­

girls, as part of a large, community based, group randomized, controlled trial of ITNs in an area of

intense perennial malaria transmission in western Kenya. To our knowledge, this is die first study

describing die impact of ITNs in adolescent girls in sub-Saharan Africa. One previous study, which

compared die efficacy of ITNs with placebo-treated bed net in two age groups was conducted in

Irian-Jaya, which has hypercndemic malaria.3S A significant reduction in infection rates and densities

of P. falciparum was found in children older than 10 years and adults mat used treated bed nets.35

In the current study, we observed a reduction in die prevalence of mild all-cause anemia

(Hemoglobin level <12 g/dL) from 3 1 % to 17% in schoolgirls 12—13 years of age. This was

associated with 0.34 g/dL higher mean hemoglobin concentrations in girls living in ITN

villages. Similarly the prevalence of mild anemia associated widi concurrent parasitemia was

reduced from 12% to 3%. This improvement in hematologic status in the 12—13-year-old girls

from ITN villages implies that malaria is still a substantial contributor to anemia in these young

adolescents and that the ITN intervention can provide important benefit to this age group.

There was no evidence of a beneficial effect on anemia in older adolescent girls 14—18

years old.This provides furdier observational support for the hypothesis that maximal expression

of resistance to infection and morbidity occurs later in adolescence and may depend on age,

a proxy for cumulative exposure, and also on host pubertal development, independent of age.10

In all of our study adolescents, regardless of age, ITNs did not appear to affect malaria pre­

valence, parasite density, all-cause morbidity, nutritional parameters, or the use of healthcare or

antimalarial medication. Other malaria intervention studies in sub-Saharan Africa have shown a

relationship between malaria and under-nutrition in young children 2>36'38 and in our study site,

ITNs resulted in improved weight gain in children less than three years old 39 and improved

weight and height gain in infants.40 A contemporary malaria intervention cohort study in pri­

mary school children 5-12 years old conducted in our same study area showed no improvement

in nutritional status.19 However, young adolescent girls experience the highest growth velocity

after infancy, and it is plausible that frequent malaria infection could have a negative impact on

such growth. We observed no evidence that ITN use resulted in improved linear growth or BMI

scores in any of the adolescent girls, including those 12-13 year olds.

Bed nets in the prevention of malaria and anemia 83

A considerable proportion of women in developing countries will have their first pregnancy

during adolescence,41 and young primigravidae are at particular risk of the adverse consequences

of malaria-associated morbidity such as severe maternal anemia and low birth weight.42

The observed improvements in hemoglobin concentrations with die ITN intervention in young

adolescents are likely to have important functional benefits for those who might become pregnant.

Conversely, while it is well understood that early teenage pregnancies, in which girls have not

reached their full growth potential, have been associated widi an increased risk of obstructed

labor due to cephalo-pelvic disproportion and its associated increased risk of peripartum maternal

mortality,43 our study suggests that antimalarial interventions provided only in adolescents may

not alter this scenario.

Several design-related limitations should be considered when interpreting the results of

our study. A caveat associated with generalization of these findings concerns die school-based

design. No attempt was made to identify adolescents either absent from school on the day of

survey (e.g. due to illness) or to evaluate the effect in children who do not attend school at all.

Girls may drop out of school if they become orphaned or because of teenage pregnancv.

The selected study sample is dius likely to be biased towards healthier girls, possibly with higher

socio-economic status, potentially resulting in an underestimation of the proportion of girls

with clinical malaria and more severe anemia. It is also noted that the effect observed in the

current study was due to a combination of individual barrier protection by ITNs, and a general

reduction in malaria transmission consequent to the observed area-wide community or mass

effect of insecticide-treated ITNs on vector populations and sporozoite rates.44 It is likely tiiat

the area-wide reductions in die malaria-transmitting mosquito populations, which benefited at

least 2 3 % of the control population in this area, will have resulted in an underestimate of the

impact of ITNs.4S Third, both the ITN trial and this school-based study were group-randomized

trials (cluster randomization), which because of the limited number of assignment units, have

a greater potential for bias dian studies which use randomization by individual.46 Furthermore,

the study in adolescent schoolgirls was conducted in preparation for a nutritional intervention

study, independently, but simultaneously with the ITN trial. It was not designed originally to

determine the impact of ITNs, hence the lack of a baseline survey to determine if the study

groups were comparable before the introduction of ITNs.

Should prevention and intervention programs directed at adolescent health in malarious

areas include ITN distribution efforts focused on this population? Our study suggests that the

prevention of malaria by permethrin-treated bed nets would halve die prevalence of mild anemia

in young, school-attending, non-pregnant adolescent girls in this area and similar areas with

intense malaria. However, it also suggests that in older girls little direct health benefits can be

expected from school-based ITN programs and would not justify diverting resources from

other essential preventive health activities. Conversely, targeting school-age children may have

indirect benefits to the community, and may be a practical means to increase ITN coverage and

contribute to any area-wide reductions in malaria transmission.47 Furthermore our analyses

presented elsewhere in this supplement show that the prevention of malaria by ITNs during

pregnancv and infancy have a marked beneficial impact on maternal health, birth outcome and

subsequent infant survival.6.40.48 However, teenage pregnant girls and their newborns are the

least likely to be ITN users.48-49 Thus, the additional public health value of such school-based

ITN programs could thus be their potential to reach and educate teenage girls on the benefits

of ITNs before they become pregnant and drop out of school. The potential direct and indirect

benefits of distributing ITNs through schools, along with other intervention programs 5 0 5 2 for

adolescent girls, deserve further study.

Acknowledgements

We express our gratitude to the schoolgirls, parents and teachers who participated in the study,

and the many people that assisted with this project. We are grateful to Dr Richard Steketee for

reviewing this manuscript. We also thank the Director of the Kenya Medical Research Institute

(KEMRI) for his permission to publish this work.

Bed nets in the prevention of malaria and anemia 85

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